Magnetic tunnel junctions MTJs are key elements in spintronic devices. In STT-MRAM and logic circuits, MTJs are used as storage elements. For sensor applications, their magnetoresistance is used to convert changes in magnetic field into changes in electrical resistance. In spin transfer oscillators, their magnetoresistance combined with spin transfer torque is used to generate RF voltages across the MTJ pillar. MTJs comprise essentially two magnetic electrode layers separated by a thin tunnel barrier that most often is made of MgO. The magnetization of one of the magnetic electrodes is fixed by a synthetic antiferromagnetic layer SAF while the magnetization of the other electrode (called free layer or storage layer SL) can be switched to parallel or antiparallel alignment with that of the fixed layer. The fixed layer is also called a polarizer layer PL. A typical structure of a MTJ is showed in FIG. 1. The switching of the SL magnetization is induced by a current of sufficiently large amplitude flowing vertically through the stack upwards or downwards by using spin transfer torque.
The magnetic state of the memory element is read out by using the tunnel magnetoresistance TMR of the MTJ. Parallel magnetic configuration usually yields a lower resistance than the antiparallel configuration. MgO based MTJs exhibit a large TMR amplitude exceeding 200% with practical perpendicular MTJ stacks at room temperature and even up to 600% in some cases. This large TMR is obtained thanks to the bcc (body cubic centered) coherent crystalline nature of both the CoFe alloy based magnetic electrodes and of the MgO barrier. This crystalline coherence yields a spin-filtering effect associated with the symmetry of the electron wave-functions. This additional spin-filtering mechanism is responsible for the large TMR of these junctions.
A stack appropriate for magnetic random access memories (MRAM) applications satisfies a number of requirements both from magnetic and transport points of view. Concerning the magnetic properties, it exhibits a strong pinning of the polarizer layer PL to make sure that its magnetization does not switch during operation as well as a high thermal stability of the storage layer SL magnetization to ensure the required memory retention but still having the ability to switch the storage layer magnetization with minimum current density. Concerning the electrical properties, the MTJ stack has a TMR amplitude as large as possible, preferably above 200% and even higher for high density memory applications (several Gbits) as well as resistance×area RA product adjusted so as to approximately match the resistance of the selection transistor in passing mode (i.e. RA product in the range of 3 to 10 Ω. μm2 but decreasing as the memory dot size decreases).
The process of deposition and annealing of MgO based magnetic tunnel junctions MTJ is well known by the man skilled in the art. An example of such a magnetic tunnel junction MTJ is shown in FIG. 1.
In this case the layers have a magnetization perpendicular to their plane. The storage layer SL consists here of 1.5 nm CoFeB/0.3 nm Ta/1.4 nmCoFeB. The storage layer SL is sandwiched between two MgO tunnel barriers to increase the perpendicular anisotropy of the storage layer arising from the CoFeB/MgO interfaces. The cap layer is here non-magnetic.
Amorphous CoFeB alloys are used as electrode material in contact with the MgO barrier. The stacks are subsequently annealed at elevated temperature (typically in the range 300-400° C.) to improve the crystallization of the MgO barrier into (100) bcc structure and induce the crystallization of the CoFeB alloy from the MgO interface towards the bulk of the layers. The CoFeB layers crystallize in a bcc structure which matches the (100) bcc structure of MgO. This results in a nice crystalline coherence between the CoFe based magnetic electrodes and the MgO barrier which is required to get a large TMR amplitude. In this bcc structure, the growth planes (parallel to the CoFeB/MgO interfaces) have a 4-fold symmetry. During recrystallization process, the B has to be expelled out of the CoFeB layer. This is achieved by introducing in the stack a B absorbing layer in direct contact with the CoFeB layer at the interface opposite to the CoFeB/MgO interface. The most widely used B absorbing layer is made of Ta but Mo, W or Hf have also been proved to work. Depending on the application, magnetic tunnel junctions MTJs can be developed with in-plane or out-of-plane magnetization. For the main application which is STT-MRAM, mostly out-of-plane magnetized MTJs are used because they offer better trade-off between thermal stability of the storage layer SL and write current than their in-plane counterpart. In these out-of-plane magnetized MTJs (also called perpendicular MTJs, noted pMTJs), the polarizer layer PL have a strong perpendicular anisotropy so that the polarizer layer keeps a stable magnetization during all the memory life time. For that, the polarizer layer PL is usually coupled through the thin B absorbing layer to another layer called pinning layer PI having strong perpendicular anisotropy (typically a (Co/X) based multilayer), where X represents Pt, Pd or Ni metals. Then, to reduce the stray field exerted by this pinning layer PI and the polarizer layer PL on the storage layer, a synthetic antiferromagnetic reference layer SAF with perpendicular anisotropy pSAF is usually used in the stack. The conventional pSAF layer in perpendicular tunnel junction stacks pMTJs comprises two sets of Co/X multilayers ML1, ML2 anti-ferromagnetically coupled through a thin Ru layer called an antiferromagnetic coupling layer (AFC) via Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Ru layer thickness is typically between 0.4 and 0.8nm to provide the antiferromagnetic coupling. These Co/X multilayers generally have a cubic face centered fcc structure and grow by sputtering in the (111) direction. As a result, the growth planes have a 3-fold symmetry. Due to the difference of symmetry order of the CoFeB/MgO/CoFeB part of the MTJ stack and of the pinning layer (4-fold mversus 3-fold symmetry respectively), a symmetry breaking layer SBL is required to allow the structural transition from one to the other structure. Ta which is used as B absorbing layer also realizes this function of structural transition layer thanks to its nano-crystallinity. The configuration of conventional pSAF in conventional pMTJs given in FIG. 1 is therefore: buffer layer/[Co/X]m/Co/Ru/[Co/X]n/Co/symmetry breaking layer/CoFeB/tunnel barrier,
where X represents Pt, Pd or Ni metals. The layers below Ru, ([Co/X]m/Co) are dubbed here as the harder layer HL and above (/[Co/X]n/Co/structural symmetry breaking layer /CoFeB) constitute the so-called softer layer. As mentioned previously, these hard layer HL and softer layer are antiferromagnetically coupled by Ru layer via RKKY interaction.
We mention that in in-plane magnetized MTJs, the same problem of structural transition between fcc and bcc parts of the stack exists. Indeed, for in-plane magnetized MTJs, the pinning of the in-plane synthetic antiferrromagnetic reference layer SAF is usually achieved by the phenomenon of exchange bias obtained by coupling one of the ferromagnetic layer of the SAF to an antiferromagnetic layer (most often made of Ir20Mn80, about 5 to 8 nm thick) which has a fcc structure. Therefore, a structural symmetry breaking layer is introduced between the pinned part of the stack and the polarizer layer to allow the structural transition from fcc to bcc. Synthetic antiferromagnetic reference layers SAF for in-plane magnetized MTJs have commonly composition of the form:
buffer layer/IrMn 7 nm/CoFe 2.5 nm/Ru 0.8 nm/CoFe 1 nm/structural symmetry breaking layer 0.3 nm/CoFeB 1.8 nm/tunnel barrier,
Using such thick conventional in-plane or out-of-plane magnetized SAF layer poses difficulty during etching of MTJs for spintronic devices fabrication. This difficulty arises due to the generation of non-volatile etch product in the etching chamber which are redeposited at the side wall of the magnetic cell and particularly aside of the tunnel barrier, providing shunting paths for current. The thicker the conventional SAF, the larger the risks of short-circuit due to redeposition of non-volatile etch products. This affects the yield, reduces the magnetoresistance amplitude, and increases the dot to dot variability. In the state of the art, in the case of out-of-plane magnetized MTJ, a thin pSAF structure was proposed and demonstrated to partially improve this problem by using a pSAF layer configuration consisting of buffer layer/[Co/X]m/Co/Ru/Co/texture breaking layer/CoFeB/MgO (see for example the patent U.S. Pat. No. 8,860,156 B2 by R. Beach et al.; “Scalable and thermally robust perpendicular magnetic tunnel junctions for STT-MRAM” by M. Gottwald et al., published in Applied Physics Letters, vol. 106, 2015).
The difference with the previously described conventional pSAF is that the layer above the Ru spacer does not contain a Co/X multilayer. It is directly the antiferromagnetic coupling with the multilayer below the Ru spacer plus the interfacial perpendicular anisotropy at the CoFeB/MgO interface which pulls the magnetization of the soft layer and polarizer layer (here Co/Ta/CoFeB) out-of-plane. However, this pSAF layer does not show sharp magnetic reversal with high squareness after annealing at 400° C. temperature. This is most likely due to interdiffusion of symmetry breaking material (for instance Ta) into the CoFeB layers. It is known that Ta indeed easily diffuses into CoFeB alloys above 300° C. preferentially along the grain boundaries during the annealing process. In the cited references, Ta and an alloy based on Fe, Co and Ta were used as symmetry breaking layer to ensure the transition from fcc (111) to bcc (001) CoFeB polarizer layer. The patent U.S. Pat. No. 8,860,156 B2 does not report anything about the annealing stability of the pSAF layers. Moreover, in the pSAF layer configuration of the cited references, the RKKY coupling layer Ru and symmetry breaking layer were separated by a Co dusting layer. To balance the offset field originated from this additional Co dusting layer on the storage layer, the magnetic moment and therefore the number of bilayers of the hard multilayer HL must also be increased. Therefore, it partially limits the thickness reduction of the pSAF.